The energy efficiency of an engine or compressor is directly proportional to the rate and volume of intake fluid drawn into a cylinder and of exhaust fluid expelled from the cylinder per stroke. The greater the flow rate of intake or exhaust fluid the greater the energy efficiency of the engine or compressor. The energy efficiency of an engine or compressor can be increased by varying the timing of the intake and exhaust values with respect to the speed of and to the load placed on the engine or compressor. Specifically, the point in time in which a valve opens or closes in relation to the position of a piston in a cylinder and the position of other valves may be adjusted to create optimal fluid flow rates. The optimal fluid flow rates vary depending on how fast the crankshaft is turning and what load is present on the engine or compressor.
Generally, an oblong cam rotatably engaged in time with a crankshaft is used to drive a push rod and rocker arm assembly to open a valve. A spring on a valve shaft closes the valve and maintains the rocker arm and push rod in contact with the rotating oblong cam. An oblong cam can also be used to drive a valve shaft directly, again employing a return spring to keep the valve shaft in contact with the cam at all times. In order to vary the timing and the length of time the valve is open, the cam diameter or attach angle must be changed responsive to the speed of the crankshaft.
Known oblong cam driven systems have several limitations. First, the combined speed of ascent and descent of the cam can only fall within a limited range. Ascent speed is limited by the mechanical connection between the cam and cam followers. If the ascent rate is too fast, shearing will occur at the cam follower surface. At high speeds, valve "float" is a problem, i.e., the valve is unable to close completely within a single full cycle of the cam. Valve float at high engine speeds occurs because the rate of closure of a valve is controlled by the stiffness of a return spring, and if the cam speed is too high, the strength of the valve return spring will be insufficient to close the valve before the cam begins a second cycle. The valve return spring must be strong enough to hold the exhaust valve shut during the intake stroke. However, if the valve return spring is too strong it will cause higher parasitic losses, strain on the valve train, and decrease energy efficiency of an engine. Reliance upon valve return springs is a second limitation in most known cam driven combustion engines.
Other mechanisms for opening and closing valves in cam driven systems are known. U.S. Pat. No. 5,078,102 to Matsumoto discloses a system wherein a rotating cam is replaced by a stepped cam plate which is disposed substantially perpendicular to the longitudinal axis of the camshaft. The sliding horizontal cam directly forces an opposing rocker arm up, thereby actuating a valve. The timing of an engine equipped with this valve opening system is changed by mechanically lengthening or shorting various mechanical control elements which change the relationship of the cam surface in response to crankshaft's angular position.
Stepped cam plate systems suffer several limitations. They are difficult to install on existing engines because the travel of the step cam plate is perpendicular to the rotational axis of the crankshaft and camshaft. These systems are also difficult to use in retrofitting existing engines, and the timing variation is accomplished by way of a complex hydraulic system which is difficult to install and maintain.
U.S. Pat. No. RE. 30,188 to Predhome, Jr. discloses a desmodromic cam and cam follower to convert rotation of a camshaft to rotary oscillation of the cam follower and in turn into activation of valves. The system is difficult to use in retrofitting existing engines and still employs return springs to close valves.
U.S. Pat. No. 5,483,929 to Kuhn et al. ("the Kuhn et al. '929 Patent") discloses a linear reciprocating camshaft having longitudinally extending cam grooves that are engaged by captive cam followers which oscillate up and down in response to sideways reciprocation of the camshaft. This device is used for operating intake and exhaust valves of machines, such as internal combustion engines or compressors, employing reciprocating pistons and valves. The camshaft is caused to reciprocate by a "yankee" type rotary-to-linear converter comprising a composite helical channel network on the surface of an end of the camshaft. The helical channel network comprises two continuous, complementary, oppositely threaded, intersecting and opposing helical channels. The helical channel network is engaged with a rotary driven collar by way of opposing triplets of radially spaced, freely rotating guide balls which engage complementary constraining slots and guide slots in the rotary driven collar. Each triplet of guide balls, which are further constrained by plural retaining clips, is engaged with its own helical channel.
Although, the reciprocating valve actuator-based system of the Kuhn et al. '929 Patent provides longer power cycles, improved energy efficiency, increased wear life, elimination of valve return springs, and increased horsepower over that provided by conventional internal combustion engines, it still possesses several disadvantages: 1) excessive wear of the reciprocating rod at turn-around points in the helical channels due to extremely high drive collar speeds; 2) guide ball breakage; and 3) complexity of the system due to an increased number of parts. Valve timing is changed by variably aligning the captive cam followers in relation to the cam grooves on the reciprocating camshafts. The shaft of each valve can be coupled to the captive cam follower so that the cam follower opens and closes the valve directly.
Given a continuing interest in the design and manufacture of energy efficient engines, there exists a need for an improved reciprocating valve actuator-based engine or compressor, and especially for one that requires lower drive collar rotational speeds, comprises less individual small components and is less prone to malfunction.